The present invention relates to an asphalt drum mixer of the type employed to continuously heat and dry stone aggregate, while mixing the heated and dried aggregate with liquid asphalt to produce asphalt paving composition.
In one conventional drum mixer of the described type, the aggregate drying and mixing steps are carried out in a rotating drum which is inclined from the horizontal. The virgin aggregate is introduced into the upper end of the drum, and an outlet is provided adjacent the lower end of the drum for withdrawing the heated and dried aggregate. Thus as the drum rotates, the aggregate cascades through the interior of the drum and moves toward the outlet at the lower end. A burner is mounted adjacent the upper end of the drum so as to create a heated gas stream which moves through the drum in a direction parallel to the moving aggregate. Also, liquid asphalt is introduced into the interior of the drum at a location midway along its length, and so that the asphalt becomes mixed with the cascading aggregate and so as to produce a paving composition which exits from the outlet. In some designs, the drum includes a center inlet by which recyclable asphalt product (RAP) may be introduced into the drum so as to be mixed with the aggregate and liquid asphalt. A drum mixer of this general type is disclosed in the U.S. Pat. No. 4,332,478, to Binz.
In another conventional drum mixer, the drum is constructed so that the heated gas flows counter to the direction of movement of the aggregate. A mixer of this type is disclosed in U.S. Pat. No. 4,867,572 to Brock et al. More particularly, in the drum mixer of the Brock et al patent, a fixed sleeve surrounds the lower portion of the rotating drum so that the heated and dried aggregate is discharged into the annular chamber which is formed between the drum and sleeve. Also, an inlet is provided in the sleeve by which RAP may be introduced into the annular chamber, and another inlet is provided to introduce liquid asphalt into the annular chamber. The drum mounts mixing blades which are positioned in the annular chamber to mix the materials and cause them to be moved longitudinally to the discharge outlet of the sleeve.
In drum mixers of the described type, it is common to utilize a filtering baghouse to remove particulate matter from the exhaust gas of the mixer. Also, it is recognized that condensation of moisture or acid in the baghouse is detrimental, in that such condensation promotes corrosion and tends to blind the filtering bags. Thus the temperature of the exhaust gas should be maintained at a minimum level to minimize such condensation. However, excessive heat is also a problem, in that it tends to destroy the filtering bags. Thus it is important that the temperature of the exhaust gas from the mixer be maintained within acceptable limits.
U.S. Pat. No. 5,052,810 to Brock discloses a drum mixer having an improved capability of maintaining the exhaust gas temperature within acceptable limits under varying operating conditions. Specifically, the '810 patent discloses the use of a gas flow bypass tube which is mounted coaxially within the downstream end of the drum. The tube may be selectively opened and closed, and upon opening the bypass tube, a portion of the heated gas is able to pass freely through a portion of the length of the tube and to the exhaust duct, without passing through the cascading aggregate. Thus the temperature of the exhaust gas rises. Upon the bypass tube being closed, all of the heated gas flows through the cascading aggregate, and the temperature of the exhaust gas is lowered.
While the bypass tube of the '810 patent represents a significant improvement, the size and length of the bypass tube has resulted in an interference with the proper showering of the cascading aggregate in the drum, and thus the heat transferred to the aggregate becomes somewhat unpredictable.
It is accordingly an object of the present invention to provide a drum mixer of the described type which is useful in the continuous production of asphalt paving composition, and which has the ability to maintain the exhaust gas temperature within acceptable limits under varying operating conditions.
It is a further object of the present invention to provide a drum mixer of the described type which does not significantly interfere with the proper showering of the cascading aggregate in the drum, and so that the heat transfer to the aggregate is predictable.